The Digestive System.

• Consists of the muscular digestive tract assisted by various accessory organs.
• Digestive functions include:
  -Ingestion.
  -Mechanical processing.
  -Digestion.
  -Secretion.
  -Absorption.
  -Excretion.
• Digestive Tract Components:
  -Oral cavity.
  -Pharynx.
  -Esophagus.
  -Stomach.
  -Small intestine.
  -Large intestine.
  -Rectum.
  -Anus.

Oral Cavity:

• Sensory analysis of potential foods.
• Mechanical processing using teeth, tongue, and palate.
• Lubrication of food by mucus in saliva.
• Enzymatic digestion by enzymes in saliva ( Salivary Amylase and Salivary Lipase).

Tongue:

• Mechanical processing of food.
• Manipulation to assist chewing and swallowing.
• Sensory analysis (taste, texture).
• Participation in speech.

Salivary Glands:

• Three pairs of glands:
  -Parotid.
  -Sublingual.
  -Submandibular.
• Functions of saliva:
  -Lubricates.
  -Cleanses oral cavity.
  -Dissolves chemicals.
  -Suppresses bacterial growth.

Movement of materials:

• Involuntary control of smooth muscle of muscularisexterna.
• Two kinds of movement:
  -Peristalsis propels material along the tract.
  -Segmentation churns material in the small intestine.

Pharynx:

• Common passageway for food, drink, and air.
• Pharyngeal muscles propel food along esophagus toward the stomach during swallowing.

Stomach:

• Temporary storage of ingested food (6-8 hours).
• Mechanical breakdown of food.
  -Forms chyme, a liquid suspension.
• Breakage of chemical bonds in food by action of acid and enzymes.
• Production of intrinsic factor.
  -Required for vitamin B12 absorption.

Gastric Glands:

• Parietal cells.
  -Secrete HCl (strong acid), intrinsic factor.
• Chief cells.
  -Produce pepsinogen, an inactive enzyme.
  -HCl activates pepsinogen to pepsin.
• Goblet cells.
  -Produce mucus.
  -Endocrine cells.
  -Produce gastrin, a hormone.

Gastric Activity:

• Cephalic phase -CNS prepares the stomach to receive food.
• Gastric phase -Begins when food enters stomach.
• Intestinal phase -Controls the pace of gastric emptying.

Key Note:

The stomach provides for the physical breakdown of food that must precede chemical digestion. Pepsin and acid begin the digestion of proteins. For a variable period of time after food arrives in the stomach, starch continues the digestion that began with salivary amylase.

Regions of the Small Intestine:

• Duodenum.
• Jejunum.
• Ileum.
  -Ileocaecal valve (a sphincter) marks junction with large intestine.

The Intestinal Wall:

• Mucosa has transverse folds, plicaecirculares.
• Plicae have small projections, villi.
• Both increase surface area of mucosa for absorption.
• Each villus has a lymphatic capillary, a lacteal.

Two types of Peristalsis:

• Small-scale periodic contractions of the muscularisexterna.
• Large-scale contractions coordinated by reflex:
  -Gastroenteric reflex.
  -Gastrocolic reflex.

Intestinal Secretions:

• Intestinal juice.
    -Moistens chyme, buffers stomach acid, dissolves digestive enzymes/ products of digestion.
• Mucus.
• Hormones.

Intestinal Hormones:

• Gastrin.
• Secretin.
• Cholecystokinin (CCK).
• Gastric Inhibitory Peptide (GIP).

Digestion in the Small Intestine.

• Most enzymatic digestion and absorption occurs in the small intestine.
• Digestive enzymes and buffers are released by:
  -Pancreas.
  -Liver.
  -Gall bladder.

Key Note:

The small intestine receives chyme from the stomach and raises its pH. It then absorbs water, ions, vitamins, and the products released from food molecules by the action of digestive enzymes produced by intestinal glands and the pancreas. 

• Endocrine secretion (hormones).
  -Insulin.
  -Glucagon.
• Exocrine secretion (digestive).
  -Water.
  -Ions.
  -Enzymes.
     -Carbohydrases., Lipases, Proteases, Nucleases.
• Chyme entering duodenum triggers hormone release.
• Hormones trigger release of pancreatic juice.
  -Secretintriggers water and bicarbonate.
  -CCK triggers enzymes Amylase, lipase, Proteases, Trypsin, chymotrypsin, carboxypeptidase:

Key Note:

The exocrine pancreas produces a mixture of buffers and enzymes essential for normal digestion. Pancreatic secretion is stimulated by hormones (secretin and CCK) released from the duodenum. 

• Largest visceral organ.
• Over 200 known functions.
• Four Lobes:
  -Right (largest by far).
  -Left.
  -Caudate.
  -Quadrate.
• Metabolic regulation.
  -Store absorbed nutrients, vitamins.
  -Release nutrients as needed.
• Hematological regulation.
  -Plasma protein production.
  -Remove old RBCs.
• Production of bile.
  -Required for fat breakdown.

• Gall bladder stores and concentrates bile for release into duodenum.
• Relaxation of the hepatopancreatic sphincter permits bile to enter small intestine.
  -CCK relaxes this sphincter.

Key Note:

The liver is the body’s center for metabolic regulation. It produces bile that will be ejected by the gallbladder into the duodenum under stimulation of CCK. Bile is essential for the efficient digestion of lipids; it emulsifies fats so  that individual lipid molecules can be readily attacked by digestive enzymes. 

• Reabsorbs water and compacts feces.
• Absorbs vitamins made by bacteria.
• Stores feces before defecation.
• Consists of three parts.
  -Cecum.
  -Colon.
  -Rectum.

Cecum:

• Collects and stores material from ileum.
• Begins process of compaction.
• Attaches to vermiform appendix.

Colon:

• Larger diameter, thinner wall than small intestine.
• Bears haustra(pouches).
• Possesses taenia coli (longitudinal bands of smooth muscle).

Rectum:

• Expandable for temporary storage of feces.
• Terminates in anal canal.
• Leads to anus.
• Circular muscle forms internal anal sphincter.
• Encircled by skeletal muscle.
• Forms external anal sphincter.

Functions:

• Absorption.
  -Water.
  -Ions.
  -Vitamins.
  -Organic wastes.
  -Bile salts.
  -Toxins.
• Bacterial growth.

Control of the Large Intestine:

• Stretching of stomach and duodenum triggers peristalsis of feces from colon into rectum.
• Sphincters control movement of feces toward the anus.
• Stretching of rectum triggers defecation reflex.
  -Release of feces requires relaxation of external anal sphincter.

Key Note:

The large intestine stores digestive wastes and reduces their volume by reabsorbing water. Bacteria that live in the large intestine are an important source of vitamins, especially vitamin K, biotin, and vitamin B5.

• Two Steps in Processing:
  -Mechanical processing to break down physical structure of foods.
  -Chemical processing to break the covalent bonds between food subunits.
      -Enzymes catalyze this.
      -Activate the hydrolysis of large food molecules.
• Carbohydrates.
  -Starches are broken down by amylases.
      -Results in di- and trisaccharides.
  -Enzymes on cell surface split them to monosaccharides.
  -Absorbed by the intestinal epithelium by facilitated diffusion or co-transport.
• Proteins.
  -Breakdown starts in stomach.
      -Pepsin and HCl produce fragments.
  -Breakdown continues in small intestine.
      -Pancreatic proteases produce small peptides.
      -Peptidases releases amino acids.
  -Intestinal epithelium absorbs amino acids into the body.

Fat absorption:

• Bile emulsifies fats to small droplets.
• Lipase hydrolyzes triglycerides into fatty acidsand monoglycerides.
• Lipid products form micelles.
• Lipids diffuse into epithelial cells.
• Triglycerides are reformed.
• Secreted as chylomicrons into lacteals.
• Transported in lymph to thoracic duct.

Vitamin Absorption:

• Fat-soluble vitamins combine with lipid micelles for absorption.
• Water-soluble vitamins diffuse across the digestive epithelium.
• Exception: vitamin B12 requires intrinsic factor (from the gastric mucosa) for absorption.

• About nine liters/day enter the digestive tract.
  -Two liters of ingested water.
  -Seven liters of watery secretions.
• Ions are absorbed by many mechanisms.
  -Sodium, calcium, chloride, bicarbonate.
• All but about 150 ml is absorbed by osmosis, as water “follows” the ions.

A complex system of motility and secretion regulation which is vital for proper function.Three types of GI reflexes:

• Local reflexes
  -Reflexes that are integrated entirely within the gut wall enteric nervous system.
  -These include reflexes that control much GI secretion, peristalsis, mixing contractions, local inhibitory effects.      
• Short reflexes from the gut -Reflexes from the gut to the preveterbral sympathetic ganglia and then back to the GItract. -These reflexes transmit signals long distances to other areas of the GI tract. -Gastrocolic reflex, Enterogastricreflex., Colonileal reflex.Ileogastric reflex.
• Long reflexes. -Reflexes form the gut to the spinal cord or brain stem and then back to the GI tract. -Vago vagal reflexes. -Pain reflexes that cause general inhibition of the entire GI tract. -Defecation reflexes. -Vomiting reflexes.

• Gastrocolic (Gastroileal) Reflex.
  -Stomach activity leads to ileocecal relaxation and increased mass movements in the colon.
  -These reflexes are mediated through both long and short nervous pathways and hormone (CCK, gastrin).
  -Most evident after first meal of the day which is often followed by urge to defecate.
  -New born children routinely  defecate after a meal.
• Enterogastric reflex. -When fat or protein chime reaches the duodenum, receptors detect and send impulses to enteric nerves of the stomach that in turn cause the inhibition of the stomacheal motility and secretion. -Delays emptying of the stomach.
• Vago vagal reflexes. -GI reflex circuits where afferent and efferent fibers of the vagus nerve coordinate responses to the gut stimuli via the dorsal vagal complex in the brain. -Controls contraction of the GI muscle layers in response to the distension of the tract by food. -Allows accommodation of large amounts of foodin the GI tract.

• Defecation reflex.
  -An intrinsic reflex mediated by the local enteric nervous system in the rectal wall.
  -To be effective it usually must be fortified by parasympathetic defecation reflex.
  -Distension of the rectum causes the internal anal sphincter to relax, which produces the urge to defecate.The external sphincter is under voluntary control.
  -Relaxation of this sphincter , coupled with contraction of the rectum and sigmoid colon, results in defecation.The rectum is usually (almost) empty. -Just before defecation mass movement in sigmoid colon fills the rectum this increases pressure leading to reflex relaxation of the inner sphincter (smooth muscle) and contraction of the outer (skeletal muscle)controlled intentionally via pudendal nerves. -Stretch receptors in the rectal wall can adapt – urge to defecate can temporarily subsided suppressed.

• Vomiting (emesis).
  -The ejection of stomach contents through the mouth.
  -Preceded by nausea, sometimes anorexia, automatic reactions( salivation, sweating, cold skin).
  -Vomiting centre is in the medulla (next to cardiovascular and respiratory centres).
  -Protective reflex against toxicity ; however longer vomiting can cause metabolic acidosis and dehydration.
  -Reverse peristalsis from the middle of the small intestines to larynx.
  -Strong contraction of abdominal muscles and diaphragm. -Relaxation, the closure of the pylorus , relaxation of the lower oesophageal sphincter and finally upper oesophageal sphincter (glottis closure, inhibition of breathing). -Forced inspiration against a closed glottis –low intrathoracic pressure , and increased abdominal pressure ( exerted by the diaphragm).